It is known that liquid mixtures can be separated into their components or that one component can be separated from a multicomponent liquid mixture by various methods. These methods include distillation, adsorption, extraction and other processes by which separation is effected through equilibrium distribution of components in different phases. It is also known that equilibrium processes for separation are subject to certain limitations. By distillation, for example, it is difficult to separate components with similar boiling temperatures or even impossible to separate azeotrope forming mixtures where complicated procedures requiring expensive equipment and causing high energy consumptions have to be used.
It is further known that liquid mixtures can be separated utilizing membrane systems and processes. Examples of such membrane processes are pervaporation or vapor permeation. In these processes the liquid mixture either in liquid form or as a saturated vapor is brought into contact with a first side (feed side) of a non-porous membrane. At least one component of the feed mixture shows a preferred permeatation through the membrane, as long as the partial vapor pressure of this component is lower at the second side (permeate side) of the membrane than at the feed side. The permeating vapor can be removed by condensation, continuous pumping, or a sweeping fluid. Various membranes are known useful for the separation of liquid mixtures in pervaporation processes, wherein the component to be separated from the mixture is generally that component which preferentially permeates the membrane.
EP-A-0 096 339, 0 307 636 and 0 442 557 as well as U.S. Pat. Nos. 4,802,988 and 4,892,661, for example, describe membranes useful for the separation of water from its mixtures with organic components, whereby the separating layer of the membrane comprises polyvinylalcohol. U.S. Pat. Nos. 4,670,146, 4,728,429 and 4,865,743 and EP-A-0 221 171 disclose ion-exchange membranes which also preferentially permeate water.
U.S. Pat. Nos. 4,590,098, 4,618,534 and 4,925,562 disclose so-called hydrophobic membranes which preferentially permeate organic components from their mixtures with water, preferably at low organic concentrations of the feed mixture. U.S. Pat. Nos. 5,039,422, 5,039,418, 5,039,417, 5,030,355, 5,019,666, 5,012,035, 4,944,880 and 4,802,987 and DE-A-2 627 629 disclose membranes useful for the separation of aromatic hydrocarbons from non-aromatic hydrocarbons.
U.S. Pat. No. 4,774,365 discloses an improved process for separation of excess methanol from ethers and C.sub.4 -C.sub.7 raffinate in the production of methyltertiary-butylether (MTBE) and tertiaryamyl-methylether (TAME).
Examples of membranes found to be suitable for this operation are those of cellulose acetate, polyvinylalcohol, polysulfone, silicon rubber, and polysubstituted acetylenes, with the preferred membranes being cellulose acetate and polyvinylalcohol. Whereas some data are provided for the separation of methanol-MTBE and methanol MTBE-hexane (50:50 b.w.)-mixtures no further details of the cellulose acetate membranes used are given. U.S. Pat. No. 4,877,529 discloses a non-porous ion-exchange membrane comprising a perfluorinated acid resin having a side acid group, which has been contacted with a quartenary ammonium salt containing hydrocarbyl groups each having less than four carbon atoms. This membrane is described to be especially useful for the separation of methanol from MTBE containing low methanol concentrations.
U.S. Pat. No. 4,960,519 discloses a method for the removal of methanol from its mixtures with oxygenated compounds selected from organic ethers, aldehydes, ketones and esters. The membrane utilized comprises non-porous separating layers of a blend of polyvinylalcohol and polyacrylic acid on a polyacrylnitrile support layer.